Add reusable Gitlab::Tracking::Helpers module

What does this MR do?

There are several controllers/concerns/modules that checks for the DNT header and the logic are the same.

To make them DRY, a new Gitlab::Tracking::Helpers module was added to encapsulate the following methods:

• dnt_enabled?
• trackable_html_request?

This module is then included in those controllers/concerns/modules and call the methods that they need.

Does this MR meet the acceptance criteria?

Conformity

• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Related to #295265 (closed)